The present invention relates to a structure of an MOS semiconductor device which is used as a power device and so forth.
In the field of the semiconductor device, the high performance, high breakdown voltage and the large current operation have been intended. The performances are rapidly improved. Among power devices in which large current can be controlled at a high breakdown voltage, the IGBT (Insulating Gate type Bipolar Transistor) and the MCT (MOS gate Control Thyristor), etc., as well as the power MOSFET have been proposed as an MOS gate type transistor in which the device drive can be carried out by a voltage, which device have been main in the power device. Particularly, in the IGBT, a remarkable technical innovation such as a low generation loss, i.e., reduction of ON voltage in the supply of a large current at the conduction of the device and the shortening of the switching time (high speed response) at the on or off for current have been advanced so that it is put in practical use. FIG. 1 shows one example of the IGBT. Since the IGBT has a structure similar to that of a power MOSFET and it is a semiconductor device using the conductivity modulation, the IGBT has a feature such as a low state voltage. The operation of the device will be described below.
In FIG. 1, an IGBT 40a is a vertical IGBT in which an n.sup.+ type buffer layer 43 and an n.sup.- type conductivity modulation layer 44 are stacked on a p.sup.+ type semiconductor substrate 42 to which a drain electrode 51 is connected and which is used as a drain layer. On the surface of the n.sup.- type conductivity modulation layer 44, a p type base layer 47 is formed by diffusion using a polycrystalline silicon (gate electrode) 52 formed on a silicon oxide film (gate oxide film) 45 as a mask. In the p type base layer 47, an n.sup.+ type source layer 48 and a p.sup.+ type contact layer 49 are formed and a source electrode 50 is connected to these n.sup.+ type source layer 48 and the p.sup.+ type contact layer 49. A polycrystalline silicon gate electrode 52 is disposed across the edge of the source layer 48, the p type base layer 47 and the surface of the n.sup.- type conductivity modulation layer 44 through the silicon oxide film 45. A source terminal 60 is connected to the source electrode 50, a drain terminal 61 is connected to the drain electrode 51, and a gate terminal 62 is connected to the gate electrode 52.
In the IGBT having such structure, when a positive drain electric potential is applied to the drain electrode 51 with respect to the source electric potential which is applied to the source electrode 51 and a positive electric potential is applied to the gate electrode 52 with respect to the source electric potential, the polarity of a surface 53 of the p type base layer 47 positioned just under the gate electrode 52 through the silicon oxide film 45 is changed to an n type inversion layer to operate as a channel. Thus, the electrons which are majority carriers, are injected into the n.sup.- type conductivity modulation layer 44 through the source electrode 50, the n.sup.+ type source layer 48 and further the n type inversion layer formed in the surface 53 of the p type base layer 47. In accordance with the flow of the electrons, holes which are minority carriers are injected from the p.sup.+ type semiconductor substrate 42 which is a drain layer. As a result, the n.sup.- type conductivity modulation layer 44 becomes a so-called state of conductivity modulation state in which electrons and holes coexist. Consequently, the IGBT 40a can be operated under a low ON state voltage.
Thus, the IGBT is a semiconductor device in which the electrons and holes similarly coexist in the conductivity modulation layer as in the thyristor and a low ON state voltage can be realized. Further, the IGBT is differentiated from the thyristor in which the device drive can be carried out by current and namely, if passing electric current is set to about zero, OFF operation is not made. Since in the IGBT, the voltage control can be conducted by an insulating gate, it is noted as a switching device to which the high frequency can be applied with a low ON voltage. As the IGBT is a bipolar mode device in which carriers of both electrons and holes coexist, the switching time itself is slow in comparison to the switching time of a unipolar mode device which uses only carriers of merely electrons, such as MOSFET. However, the turn-off time has been shortened by introduction of the life time killer, etc. As described above, the IGBT is superior to the thyristor in that the generation loss can be controlled by an MOSFET and that it has a high switching speed. Thus, the IGBT is noted as a device in which a low ON as in the thyristor can be realized.
One of the most important key technology in solving the problems such as high efficiency, miniaturization, low cost, etc., in the power electronics, is to decrease a loss of the power device. Thus, the development of the power device in which the turn-off time is short and at the same time, the ON voltage is low, is required. Consequently, in the above-mentioned IGBT 40a, to more decrease the ON voltage is needed. However, the IGBT 40a is a semiconductor device in which the base current of the pnp transistor consisting of the built-in p.sup.+ type semiconductor substrate 42 which is a drain layer, the n.sup.- type conductivity modulation layer 44 and the p type base layer 47, is supplied from a MOSFET which is controlled by the gate electrode 52. Thus, the ON voltage of the IGBT 40a cannot be lowered to a level below the ON voltage of the pnp transistor. Further, the increase of the ON voltage by the JFET effect in passing the MOSFET portion formed in the IGBT 40a cannot be ignored. Namely, in the IGBT 40a, electrons are supplied from the n.sup.+ type source layer 48 to the n.sup.- base layer 44 through the n type inversion layer formed on the surface 53 of the p type base layer 47 and the n.sup.- type base layer 44 has a state of the conductivity modulation to decrease resistance. In the IGBT 40a, the pn junction between the n.sup.+ type source layer 48 and the p type base layer 47 is maintained, which is differentiated from the thyristor (in the thyristor structure, the pn junction is broken). Consequently, the electron current flows in the n type inversion layer of the surface 53 and the hole current flows in one-sided way along the n type inversion layer by the JFET effect. Thus, the IGBT 40a has limits in reduction of the ON resistance due to the channel resistance and the increase of resistance by the JFET effect. As explained above, the IGBT is a semiconductor device having such a large merit that the turn-off and the turn-on can be carried out by using the MOSFET. Nevertheless, the IGBT is a device having the above-mentioned basic problems and it has limits in the decrease of the ON voltage.
On the other hand, from the viewpoint of the decrease of the ON voltage, the ON voltage can be further decreased by adopting a semiconductor device having a thyristor structure. However, in the semiconductor device having a thyristor structure, the device drive is carried out by the current, so that the turn-off cannot be easily carried out and the shortening of the turn-off time is difficult. Thus, the realization of the power device having a required property is difficult. Although a MOS gate type thyristor device has been proposed, the turn-off proof is low and the realization of the low ON resistance in the MOSFET is needed. Solving the problem is also difficult as in the problem in the IGBT.